1. Field of the Invention
The present invention relates to a process and apparatus for manufacturing an electret article. According to the present invention, a dielectric article can be efficiently polarized to obtain, for example, a surface-charged electret article, a surface of which is considerably hetero-charged, namely, charged with both positive and negative ions; or a piezoelectric or pyroelectric electret article.
2. Description of the Related Art
Hitherto, a direct current (DC) corona charging process was generally used in the manufacture of a surface-charged electret article, such as electret fibrous sheet, as disclosed in, for example, Japanese Examined Patent Publications No. 3-54620, No. 4-8539, and No. 5-83283. In the conventional process, as shown in FIG. 1, an electret treatment of a fibrous sheet 1 was carried out by bringing the fibrous sheet 1 to be treated into contact with an electrically grounded electrode 8, such as a stainless steel drum or water electrode, and applying a high DC voltage between the grounded electrode 8 and a discharge electrode 9, such as a wire or needle electrode to thereby perform an electret treatment of the fibrous sheet 1 with a DC corona discharge.
In the resulting electret fibrous sheet, charges are maintained in the form of polarized charges within the fibers of the sheet. The surface to which the high DC voltage was applied is generally charged with the polarity same as the applied polarity, whereas the surface which is brought into contact with the grounded electrode is charged with the polarity opposite to the applied polarity. However, the degree and duration of the charging was not satisfactory.
The DC corona discharge used in the electret treatment is based on a considerably unequal electric field generated between the needle electrode 9 and plane electrode 8 shown in FIG. 1. It is necessary to produce more than a certain degree of an electric intensity between the needle electrode 9 and plane electrode 8, namely, a voltage for initiating the corona discharge or more, to generate the corona discharge. The wide distance between the needle electrode 9 and the plane electrode 8 requires the higher initiating voltage.
For example, when the article to be treated is charged by the corona discharge, and the potential difference between the surface of the article and the corona discharge electrode (needle electrode) 9 becomes lower than the initiating voltage of the corona discharge, the corona discharge ceases. This is because the discharge electrode is always associated with the counter electrode (grounded electrode) in the DC corona discharge. It is necessary to increase the voltage, if the surface potential, i.e., the surface charge, on the article 1 to be treated is needed to be higher. However, if the voltage becomes too high, a spark discharge may occur between the electrodes through weak portions of the article to be treated, to thereby damage the article, for example, by boring holes.
Further, when a thick non-woven fabric, such as a felt, is treated, the surface confronting the discharge electrode 9 is not sufficiently charged, and thus, the opposite-polarity charges are not injected from the grounded electrode into the opposite surface. Therefore, the felt cannot be sufficiently charged as a whole. When the resulting felt was used as a filter, satisfactory effect was not obtained. [See "Seidenki-Gakkai-Shi (Journal of Electrostatics Association)", Vol. 18, No. 2 (1994), pp. 119 to 127; and Vol. 18, No. 5 (1994), pp. 444 to 448.]
Further, in the conventional process using DC corona discharge, it is necessary to bring the article, such as a fibrous sheet, into intimate contact with a surface of the plane electrode, such as a stainless steel drum. Therefore, non-planar articles, such as a face mask or a pleated fibrous product, were not able to be charged after shaping procedures. Hitherto, the shaping procedures were carried out after charging the starting fibrous materials. There was a defect that charges were lost during the shaping procedures.
The effect of the electret treatment is gradually lost when the electret article is used for a long time, and sometimes, the electret treatment is desired again. If the article has a nonplanar structure, it is very difficult to repeat the electret re-treatment in the conventional process.
The electret articles exhibiting piezoelectric effect, namely, piezoelectric electret articles, are used in, for example, acoustic elements or displacement elements, using piezoelectric property. The conventional process of producing such a piezoelectric electret article comprises bringing the surface of the article to be treated into contact with an electrode, and applying a high voltage.
When the piezoelectric electret article is produced, generally, the article can be easily polarized by applying a high voltage to the starting article, or by treating the starting article at a high temperature below Curie temperature. The Curie temperature means the temperature where a phase transition occurs, and a piezoelectric effect which have appeared below Curie temperature will disappear above Curie temperature.
If a DC voltage higher than a certain voltage is applied between electrodes in the air, a discharge may occur between electrodes at the edge of the article to cause dielectric breakdown (insulation breakdown) of the air. When the dielectric breakdown occurs, a high voltage cannot be maintained. Thus, the polarization effectiveness is extremely lowered. If the temperature of the article to be treated is elevated by a heater or infrared radiation, the article can be easily polarized by applying a high DC voltage in the air. However, the temperature of the air is also elevated, and the dielectric breakdown of the air easily occurs.
Therefore, as shown in FIG. 2, the dielectric article 2 to be treated was hitherto polarized by attaching, to the surfaces of the dielectric article 2 which have been formed into a predetermined shape, a pair of electrodes 3a, 3b having surface areas narrower than those of the dielectric article 2; and applying a high DC voltage between the electrodes 3a, 3b, while dipping the dielectric article 2 and the electrodes 3a, 3b in an insulating oil 4. For example, a ceramic material as the dielectric article 2 was polarized by sintering a green sheet, coating a silver paste on both sides, baking the coated sheet to form the electrodes 3a, 3b on the surfaces of the dielectric article 2, dipping the dielectric article 2 together with the electrodes 3a, 3b in the insulating oil 4, and applying a high DC voltage while avoiding the dielectric breakdown of the air. The treatment is carried out at about 100.degree. C. by heating the insulating oil 4 so as to enhance polarization effectiveness. It is difficult to further raise the temperature of the insulating oil 4. Therefore, the conventional process can avoid the dielectric breakdown, but had a defect that polarization effectiveness was still poor.
In the conventional process as shown in FIG. 2, it is necessary to attach the electrodes 3a, 3b to the surface of the dielectric article 2. Therefore, there was another defect that the dielectric breakdown was liable to occur in the dielectric article 2. This is because if the dielectric article 2 has the finely rugged or rough surfaces, and then, the electrodes 3a, 3b have the corresponding finely rugged or rough surfaces, the electric intensity was sometimes concentrated in the fine recessions to cause the dielectric breakdown. Further, if the dielectric breakdown occurred by other causes, the charges accumulated in the electrodes which were closely attached were sometimes immediately poured into the dielectric breakdown portion all together to damage the dielectric article. After the dielectric breakdown occurred, the voltage was not raised, and the damage became severer. The dielectric article having the wider surfaces to be polarized provides not only a higher possibility of the dielectric breakdown, but also a larger amount of the accumulated charges poured into the dielectric breakdown portion. Therefore, it was difficult to treat the dielectric article having the wide surfaces to be polarized. The shape of the dielectric article to be treated was limited, because it was necessary to attach the electrodes thereto. Further, the conventional process required the attachment of the electrodes to the article to be treated and dipping into the insulating oil, and thus, the procedure was troublesome and was not suitable to a continuous treatment.